This application claims the priority of Korean Patent Applications No. 2001-73289, filed Nov. 23, 2001 and No. 2002-71399, filed Nov. 16, 2002 in the Korean Intellectual Property Office, which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a composite for paste, an electron emitting device using the same, and a manufacturing method thereof used in a field emission display (FED), and more particularly, to a composite for paste using carbon nanotubes (CNT), an electron emitting device, and a manufacturing method thereof.
2. Description of the Related Art
Electron emitting devices are used as sources of electron emission in field emission displays, which are attracting attention as next generation flat display devices. FEDs have merits of a high picture quality, high efficiency, and low power consumption.
The performance of the FED depends on a processing technology and the stability of the electron emitting device.
In a conventional electron emitting device using a metal tip, polymers attached for sealing during the production of a vacuum tube having the electron emitting device are not completely burned. The polymers remain in the vacuum tube and are slowly emitted with the operation of the FED. Then, the polymers are adsorbed on the surface of the metal. Therefore, the number of electrons emitted from the metal tip is reduced, thereby degrading the overall performance of the FED. Moreover, the polymers oxidize the metal tip, thereby shortening the life of the metal tip.
To solve the above-described problems, attempts have been made to use carbon nanotubes (CNTs), which have a high electron emission rate and stability, for the electron emitting device.
In a CNT, carbon atoms form the structure of a honeycomb pattern and graphite surfaces rolled with a nano-size diameter. Due to the intrinsic physical, electrical, and chemical characteristics of CNTs, CNTs have recently been used in advanced technologies.
Methods for producing the electron emitting device using CNTs, in the display technique field include: plasma enhanced chemical vapor deposition (PECVD) disclosed in U.S. Pat. No. 6,232,706; a method in which a paste is used, disclosed in U.S. Pat. No. 6,239,547; and a method in which an electric swing method is used, disclosed in Korean Patent No. 2001-0017543.
In PECVD, acetylene gas is injected into a space between two electrodes in a reactor including a nickel catalyst, and glow discharges the gas by a direct current or a radio frequency field. Then, the gas is converted into plasma for growing CNTs on the electrodes by using the conversion energy.
In the method for producing an electron emitting device using paste, CNTs are converted into CNT powder using laser ablation or an arc discharge. Then, the CNT powder is mixed with conductive or non-conductive paste for printing.
In the method for producing an electron emitting device using the electric swing method, CNTs dispersed in an aqueous solution are grown on an electrode using the electric swing method.
FIG. 1 shows an electron emitting device using the electric swing method which is disclosed in Korean Patent No. 2001-17543. Referring to FIG. 1 a conventional electron emitting device comprises a cathode 12 formed on a substrate 11; a thin film 12a attached to the cathode 12, on which tips 15 formed of CNT powder are formed; a gate insulating layer 13 for surrounding the tips 15; and a gate electrode 14 on the gate insulating layer 13 and above the tips 15, the gate electrode having an opening 14a for emitting electrons.
The tips 15 of the electron emitting device are formed by the electric swing method. Here, the thin film 12a and an electrode plate are installed in a solution of CNT powder, then the cathode of an external power source is connected to the thin film 12a, and the anode of the external power source to the electrode plate. When voltage is applied to the cathode and the anode, particles of the CNT powder, which are charged positive, are attached to the thin film 12a which is charged by the cathode. Alternatively, the substrate 11 can be substituted for the thin film 12a, and the CNT can be grown directly on the substrate 11.
In the conventional PECVD, CNTs are vertically aligned on the substrate, however it is difficult to uniformly emit electrons from a large area. Additionally, CNTs are grown in a high temperature of over 500 to 600xc2x0 C., thereby increasing the production cost, since a silicon or crystal glass substrate must be used instead of a glass substrate in order to increase the temperature of the substrate.
In the conventional method for producing the electron emitting device using the paste, the CNT powder is mixed with silver paste or a polymer compound, and a thermal process is performed at a high temperature of about 350 to 500xc2x0 C. to oxidize the CNT and the metal. Therefore, the lives of the CNTs and the metal apparatus are shortened. Moreover, the thermal processing time in producing the electron emitting device is lengthened, and a large amount of residual gas is generated by residual polymers, thereby shortening the life of the electron emitting device.
Accordingly, to solve the above-described problems, it is an objective of the present invention to provide a composite for forming an electron emitting device for uniformly emitting electrons from a large area while having excellent stability and durability, and an electron emitting device using the composite and a manufacturing method thereof.
To accomplish the objective, the present invention provides a composite for paste including 5 to 40 parts by weight of carbon nanotubes (CNTs), 5 to 50 parts by weight of alkali metal silicate, and 1 to 20 parts by weight of binder.
It is preferable that the composite further includes 5 to 40 parts by weight of graphite.
The composite formed of CNTs and the alkali metal silicate, or the composite formed of CNTs, the alkali metal silicate, and the graphite preferably also includes 10 to 40 parts by weight of water.
It is preferable that the composite further includes an additive (1 to 6 parts by weight), so that the hydrogen-ion concentration (pH) of the composite is from 10 to 14. The additive is any one of potassium hydroxide, sodium hydroxide, or ammonium hydroxide aqueous solution.
The size of the CNTs is preferably from 10 nanometers to 10 micrometers.
It is preferable that the alkali metal silicate is Na2O-nSiO2 or K2O-nSiO2, and n is from 2.2 to 3.9.
The size of the graphite is preferably from 100 nanometers to 5 micrometers.
The binder may be any one material from an organic carboxylic acid group, an organic sulfonic acid group, an ester group, an inorganic acid group, a salt group thereof, a hydracid salt, and an organic acid compound group.
It is preferable that the binder is formed by mixing at least two materials from the organic carboxylic acid group, the organic sulfonic acid group, the ester group, the inorganic acid group, the hydracid salt, and the organic acid compound group.
To accomplish the above-described objective, the present invention provides an electron emitting device including a substrate, cathode electrodes patterned on the substrate in a predetermined shape, a resistive layer stacked to encompass the cathode electrodes, thereby exposing portions of the cathode electrodes, a gate insulating layer patterned on the resistive layer in a predetermined shape to arrange wells in which the exposed cathode electrodes are located, and gate electrodes arranged on the gate insulating layer, the electron emitting device comprising electron emitting tips formed of a composite for paste including 5 to 40 parts by weight of CNTs, 5 to 50 parts by weight of alkali metal silicate, and 1 to 20 parts by weight of binder.
To accomplish the above-described objective, the present invention provides first step of forming cathode electrodes on a substrate and patterning the cathode electrodes in a predetermined shape, second step of sequentially depositing and patterning a resistive layer, an insulating layer, and gate electrodes on the cathode electrodes to form wells, which exposes portions of the cathode electrodes, third step of forming a photoresist on the sidewalls of the resistive layer, the insulating layer, and the gate electrodes, and on the gate electrodes, fourth step of printing a composite for paste including 5 to 40 parts by weight of CNTs, 5 to 50 parts by weight of alkali metal silicate, and 1 to 20 parts by weight of binder in order to coat the cathode electrodes exposed through the wells and the photoresist, and fifth step of removing the photoresist to remain the composite for paste on the cathode electrodes so as to form electron emitting tips.
It is preferable that fourth step further includes drying the composite for paste at a temperature over 50xc2x0 C. after printing the composite for paste.
The composite for paste includes 5 to 40 parts by weight of CNTs, 5 to 50 parts by weight of the alkali metal silicate, and 1 to 20 parts by weight of a binder.
It is preferable that the composite for paste further includes 5 to 40 parts by weight of graphite.
It is preferable that the composite for paste further includes 10 to 40 parts by weight of water.
It is preferable that the composite for paste further includes 1 to 6 parts by weight of additive to control the pH of the composite to be from 10 to 14.
Here, the additive is any one of potassium hydroxide, sodium hydroxide, or ammonium hydroxide aqueous solution.
It is preferable that the size of the CNT is from 10 nanometers to 10 micrometers, and the alkali metal silicate is Na2O-nSiO2 or K2O-nSiO2, wherein n is from 2.2 to 3.9.
It is still preferable that the size of the graphite is from 100 nanometers to 5 micrometers.
The binder may be any one material from an organic carboxylic acid group, an organic sulfonic acid group, an ester group, an inorganic acid group, a salt group thereof, a hydracid salt, and an organic acid compound group.
It is preferable that the binder is formed by mixing at least two materials from the organic carboxylic acid group, the organic sulfonic acid group, the ester group, the inorganic acid group, the salt group thereof, and the organic acid compound group.
The cathode electrode is formed of a conductive transparent material, preferably an indium-tin-oxide layer.
The resistive layer is amorphous silicon.
The gate insulating layer is formed of silicon dioxide, and the gate electrode is formed of chrome.
The present invention uses an electrode as an electron emitting device, which is formed by performing a photo lithography process on CNTs, or a CNT and alkali metal silicate formed by mixing CNTs and graphite with the alkali metal silicate, or the composite using CNTs, graphite, and the alkali metal silicate. Here, the CNT is operated at a lower voltage than a conventional field emission display (FED), and has a structure similar to C60, which allows the CNT to have excellent durability and electrical conductivity.
The electron emitting device produced by the composite provides a uniform electron emitting characteristic from a large area, and provides improved life and stability.